Electromagnetic Waves: Important Questions

Very Short Answer Questions (1 Mark Questions)

Ques 1. Mention the significance of the Ozone layer in the atmosphere?

Ans. Ozone Layer absorbs all the harmful ultraviolet rays and protects the earth from their ill effects.

Ques 2. Which is that part of the electromagnetic spectrum that is used in the greenhouse to keep the plants warm?

Ans. Infrared rays are used to keep the plants warm. Infrared rays absorb the greenhouse gases in the atmosphere more compared to UV rays, X rays and radio waves, and cause the temperature of the earth to rise, resulting in a greenhouse effect. That is why it is used in the greenhouse to keep the plants warm.

Ques 3. Which is that part of the electromagnetic spectrum that has the largest penetrating power?

Ans. Y rays are the electromagnetic waves which have the largest penetrating power. Its frequency ranges between 3 × 10⁸ Hz to 5 × 10²² Hz.

Ques 4. A plane electromagnetic wave travels in a vacuum along Y Direction. What is the conclusion about the direction of electric and magnetic field vectors?

Ans. The electric field and magnetic field of the electromagnetic wave which travels along the Y direction will be in the X direction and Z direction respectively.

Ques 5. Two waves are given, the first one is microwaves of wavelength 1 mm and the second is UV radiations of 1600 A⁰ in a vacuum. Name the physical quantity which remains identical for both of them.

Ans. The velocity or speed of light remains the same in the vacuum for both microwaves and UV radiations. 

Ques 6. In order to protect their eyes from electromagnetic radiation, welders wear some special goggles or face masks. What is this radiation? Write its frequency.

Ans. The radiation is ultraviolet radiation.

The frequency range of UV rays is 10¹⁵ Hz to 10¹⁷ Hz.

Ques 7. Which constituent radiation of the electromagnetic spectrum is used for aircraft navigation?

Ans. For aircraft, navigation microwaves are used.

Ques 8. Where does the speed of plane electromagnetic waves be maximum?

Ans. The speed of electromagnetic waves is maximum in a vacuum.

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Short Answer Questions (2 Marks Questions)

Ques 1. Write two applications of infrared radiations.

Ans.

The uses of infrared radiations are

1. They are employed to expose the old walls' secret writings.
2. Infrared radiations are used to photograph in hazy situations.

Ques. 2 (a) What do you mean by displacement current?

(b) What was the purpose of introducing this concept?

Ans.

1. Displacement current is the rate of change of electric displacement field. Its unit is Ampere(A) and it is the same as that of electric density. 
2. This concept of displacement current was introduced to make the Ampere-circuit law logically consistent.

Ques 3. Electromagnetic wave X₁ has a 1cm wavelength and another wave X₂ has a 10¹⁵ Hz frequency. Name the type of waves and write two applications of the two in daily life.

Ans. The name of the X₁ wave is Microwave.

Application- It is used in air navigators, microwaves etc.

The name of the X₂ wave is Ultraviolet wave.

Application- UV rays are used for food preservation and in sterilizing surgical instruments.

Ques 4. How are electromagnetic waves produced? Draw the sketch of a plane electromagnetic wave propagating along the x-axis depicting the directions of the oscillating electric and magnetic field.

Ans. Electromagnetic waves are produced as a result of oscillating or accelerating charged particles.

The sketch of a plane electromagnetic wave propagating along the X- axis depicting the directions of oscillating electric and magnetic fields is shown below.

Ques 5. Skywaves are not used in the transmission of TV signals. What is the reason? Mention two methods by which TV transmission can be increased.

Ans. Skywaves are not reflected by the ionosphere. Only the ways that are reflected by the ionosphere can be used for TV transmission. That is why it is not used for transmitting TV signals.

The two methods of increasing the range of TV transmission are done through:

1. Tall antenna
2. Geostationary Satellites

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Long Answer Questions (3 Marks Questions)

Ques 1. A capacitor, made of two parallel plates each of plate area A and separation d, is being charged by an external ac source. Show that the displacement current inside the capacitor is the same as the current charging the capacitor. (All India 2012)

Ans. The displacement current produces due to varying electric field is given by

\(\mathrm{I}_{\mathrm{D}}=\mathrm{E}_{\mathrm{D}}\left(\frac{d \phi}{d t}\right)\)

If q is the instantaneous charge, E is the electric field between the plates of capacitor at a specific period and A is the area of the plates, then,

Ques 2. The oscillating magnetic field in a plane electromagnetic wave is given by

B_y = (8 × 10^-6) sin [2 × 10^-11 t + 300 π x] T

1. Calculate the wavelength of the electo-magnetic wave.
2. Write down the expression for the oscillating electric field. (Delhi 2008)

Ans. Given that,

B_y = 8 × 10⁶ sin [2 × 10¹¹ t + 300 π x] T

The standard equation is,

The oscillations of fields E and B are perpendicular to each other. It is also perpendicular to the direction of propagation of the wave. The oscillating magnetic field is in the y-direction and propagation of the wave is in the x-direction, so take the electric field in the z-direction.

Ques 3. How does a charge q oscillating at a certain frequency produce electromagnetic waves? Sketch a schematic diagram depicting electric and magnetic fields for an electromagnetic wave propagating along the Z-direction. (Delhi 2009)

Ans.

As the charge q is oscillating, the electric field and magnetic field produced will change. It changes space and time. E and B vary with time produced by the other fields B and E respectively and sustain the E.M. pattern.

This interpretation of Maxwell is supported by the following

Ques 4. Answer the following questions.

(a) Assume that the earth doesn't have an atmosphere. Will it affect the average surface temperature of the earth?

(b) For the survival of human beings, the presence of the ozone layer on the top of the stratosphere is very significant. Substantiate this statement.

(c) It is said that X-ray astronomy is possible only from satellites orbiting the earth. Why?

Ans.

1. The phenomenon of the greenhouse effect is only possible due to the presence of the atmosphere. If the atmosphere is not there, the greenhouse effect will not be there. Because of this, there would be a decrease in the temperature of the earth.
2. It is the ozone layer that prevents the entry of harmful ultraviolet rays from the sun. It protects the life forms on earth from the detrimental effects of UV rays.
3. It is because when X-rays pass through the atmosphere, it will absorb the X-rays.

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Very Long Answer Questions (5 Marks Questions)

Ques 1. Name the following parts of the electromagnetic spectrums.

1. used in radar systems for aircraft navigation
2. used to treat muscular strain
3. used in hospitals for diagnosing diseases

Also, briefly describe how these waves can be produced.

Ans.

1. Microwaves are used in radar systems that are used in aircraft navigation. These rays can be produced by making use of special vacuum tubes, such as Klystrons, magnetrons and Gunn diodes etc.
2. Infrared waves are used in the treatment of muscular strain. These rays can be produced by hot bodies and molecules.
3. X rays are used as a very significant diagnostic tool in medicine. These rays are produced when high energy electrons hit any metal of a greater atomic number.

Ques 2. Answer the following 

1. Name the EM waves which are suitable for radar systems used in aircraft navigation. Write the range of frequency of these waves.
2. If the earth did not have an atmosphere, would its average surface temperature be higher or lower than what it is now? Explain.
3. An EM wave exerts pressure on the surface on which it is incident. Justify. (Delhi 2014)

Ans.

1. The waves which are used in radar systems are microwaves. Its frequency ranges between 10¹⁰ and 10¹² Hz 
2. If there is no atmosphere, there would be no ozone layer to prevent ultraviolet radiations reaching the earth. The temperature on earth’s surface would have been lower because there would be no greenhouse effect, and it would make human survival difficult. 
3. Since an electromagnetic wave carries both energy and momentum, it exerts pressure on the surface on which it is incident. But the pressure will be negligibly small. It is because of the extremely small momentum of the photon which can be calculated by the de-Broglie relation,\(\left(\lambda=\frac{h}{p}\right)\)or
   \(\begin{aligned} p &=\frac{h}{\lambda}=\frac{6.63 \times 10^{-34}}{10^{-9}} \\ &=6.63 \times 10^{-25} \mathrm{~kg} \mathrm{~ms} \end{aligned}\)

Ques 3. A capacitor is connected in series to an ammeter across a d.c. source. 

1. Why does the ammeter show a momentary deflection during the charging of the capacitor? What would be the deflection when it is fully charged?
2. How is the generalized form of Ampere’s circuital law obtained to include the term due to displacement current? (Comptt. All India 2014)

Ans. 

1. The ammeter shows a momentary deflection, because of the fact that when the capacitor is charged there will be the flow of the transient through the circuit. But, when the capacitor gets fully charged the deflection becomes zero.
2. Maxwell’s displacement current

Let the magnetic field be, B and the current be I,

According to Ampere’s circuital law, the magnetic field is related to the steady current.

Maxwell proved that this relation is not logically consistent. He accounted this inconsistency as the following:

Ampere’s circuital law for loop C₁ =

which is not logically consistent. 

That is why Maxwell put forward the idea of displacement current.

As Maxwell opined, displacement current is the current which comes into contact to the region where the electric field.

That is, the electric flux is always changing according to time.

It is now known as Ampere-Maxwell law. 

Given above is the generalization of Ampere’s circuital law.

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